Coronary artery disease is a leading cause of mortality in the western world. Percutaneous coronary interventions (PCI) are a current mainstay of therapy. In 2001, an estimated 1.051 million PCI were performed in the United States (1). Procedural success rates in stenotic (but non-occluded) coronary artery lesions are in excess of 95%. However, in selected lesion subgroups, such as chronic total occlusions (CTO), calcified stenosis and non-compliant plaques, procedural success rates are considerably lower. In CTO, success rates are in the range of 60 to 70% range (2-4), despite continuing improvements in angioplasty technology (5,6). This current success rate for CTO may even be an overestimation, since a significant portion of CTO crossings are not attempted due to low expectation of success. Inability to cross the CTO with a guide-wire is responsible for 75% of PCI failures (4,7). In certain cases, the balloon or stent cannot cross the lesion despite successful guide-wire crossing.
A chronic total occlusion is an occlusion that is greater than a month old. CTOs are commonly found in patients undergoing diagnostic coronary artery catheterisation with up to 20-33% of patients reported to have one or more CTO (8,9). This includes a large number of patients that have not experienced myocardial infarction. Successful revascularization of a CTO significantly improves angina in symptomatic patients (10,11) and more recent data suggest improvement in left ventricular function (12-16), and even in reduction of mortality (17-19). Currently, there are two major therapeutic strategies for addressing problems associated with CTOs: coronary artery bypass graft surgery (CABG) or percutaneous coronary interventions (angioplasty or stenting). Angioplasty includes placement of a small (360 μm diameter) guide-wire through the tissue obstructing the lumen in a CTO in order to reach the distal arterial lumen. The technical difficulty of performing PCI in CTO, primarily because of inability to cross CTO with a guide-wire, is reflected in the low rates of PCI for CTO (accounts for less than 8% of all PCI), despite the benefits of a positive outcome (20). Since PCI have severe limitations in this patient subset, clinicians frequently decide to refer these patients for CABG or persist with (often ineffective) medical therapy. The presence of one or more CTO in vessels supplying viable myocardium remains one of the most common reasons for referral for CABG rather than attempting PCI (21).
Several lesion characteristics have been identified as predictors of procedural success and influence the decision to proceed with angioplasty. Duration of occlusion, which is often difficult to ascertain, is a major predictor. Recent coronary occlusions, i.e. less than three months old, are successfully dilated 74%-89% (4, 22) of the time. However, success rates in occlusions greater than three months old decline to 45%-59%. Other predictors of procedural failure include occlusion length (greater than 15 mm) (6,7,9), presence of bridging collaterals, and an absence of a tapered funnel leading into the occluded segment (23). Failure rates are also higher in absolute occlusions (no distal opacification) than in functional total occlusions (subtotal occlusion with faint late anterograde opacification of the distal segment without discernible continuity) (9,24,25).
Inability to cross the CTO with a guide-wire is responsible for 75% of PCI failures (4,7). In some cases, the balloon or stent cannot cross the lesion despite successful guide-wire crossing.
Successful angioplasty requires that the operator first place a small (0.014″ or 360 μm diameter) guide-wire through the narrowed lumen of a lesion in order to reach the distal arterial lumen. This is followed by placing a balloon angioplasty catheter or coronary stent mounted on a balloon catheter across the lesion, then dilating the balloon to expand the lesion or deploy the stent. The cross-sectional size of balloon angioplasty catheters varies. A fixed-wire balloon catheter, e.g. ACE, Boston Scientific, typically has a diameter of about 610 μm. A rapid exchange balloon, e.g. Maestro balloon-check 1.5 mm balloon, typically has a 900 μm diameter. Over-the-wire (OTW) balloon catheters typically have a 1100 μm diameter, e.g., Opensail of Guidant Corp. There are different strategies to overcome this limitation. Recently it has been reported that a 0.9 mm high-energy excimer pulsed laser catheter (X80, Spectranetics, Colorado Springs, Colo.) had a success rate of 92% in crossing calcified and/or balloon-resistant lesions that could not be crossed with a 1.5 mm diameter balloon catheter (26,27). However, the laser system is expensive and access is limited to a few clinical sites. Rotational atherectomy has also been used in such situations but first requires crossing the lesion with a rota-wire, which is more difficult to manipulate than conventional guide-wires, particularly in a chronic total occlusion.
The underlying atherosclerotic plaques in chronic total occlusions are predominantly fibrocalcific (28), consisting of smooth muscle cells, extracellular matrix, calcium and variable amounts of intracellular and extracellular lipids (29). Inflammatory cells are commonly seen (28). Collagens are the major structural components of the extracellular matrix, comprising up to 50% of the dry weight (30,31), with predominance of types I and II (and minor amounts of IV, V and VI) in the fibrous stroma of atherosclerotic plaques (32,33). In CTO less than 1 year old, proteoglycans are also commonly found in the intima. Thrombus formation contributes to a varying degree, depending on the severity of the underlying atherosclerotic plaque, and can result in single or multiple layers of clot. Over time, this thrombus becomes organized and converted into a collagen-rich fibrous tissue (known as intimal hyperplasia), which eventually is incorporated into the underlying atherosclerotic plaque (29). Older, more organized collagen-rich fibrous tissue, particularly with high calcium content, appears to be the barrier to successful crossing with current angioplasty techniques of both the guide-wire and the balloon angioplasty or stent catheter. In severely stenosed but non-occluded arterial lesions, particularly if heavily calcified and fibrotic, operators also may encounter difficulties in crossing the balloon angioplasty or stent catheters, even when the guide-wire has successfully crossed.
The literature contains descriptions of a penetration catheter (Tornus, Sahi Intecc, Aichi, Japan) (34). The Tornus catheter consists of a three parts: the main shaft with surface coating, the polymer sleeve and a hub connector. The main shaft is a coreless, stainless steel coil that is right-handed lay (clockwise). Eight stainless wires are stranded in the coil. The outside diameter is 0.70 mm. The inside diameter is 0.46 mm and is suitable for the 0.014″ guide-wire. The device is advanced across severe stenosis by rotating the guide-wire counterclockwise rotation, the shaft being stranded clockwise. The profile of the tip is 0.62 mm in diameter and it is made of stainless-platinum alloy. Tsuchikane et al. have described the initial use of this device in patients with severe coronary artery disease or calcified stenosis when a 1.5 mm balloon catheter or microcatheter did not cross through the lesion after successful wire crossing. In 14 patients, the Tornus was successfully crossed through all lesions without distal embolism, coronary perforation or dissection. Stent implantation was successfully performed in all 14 patients.
In summary, percutaneous coronary interventions are an important form of coronary artery revascularization therapy. In most cases, crossing the lesion with guide-wires and angioplasty and stent catheters is quite straightforward. However, in a particular subgroup of coronary lesions, e.g., chronic total occlusions or heavily calcified, non-compliant stenoses, crossings with balloon angioplasty catheters and stent catheters remain a challenge despite successful crossing with a guide-wire.
There is thus a need to be able to treat lesions to facilitate balloon angioplasty or stent catheter passage across an arterial occlusion or stenosis after the guide-wire has successfully crossed.